In the preparation of long-chain polyamides, ordinarily a salt solution of a dicarboxylic acid and a diamine is heated until the diamine-dicarboxylic acid salt begins polymerizing by polycondensation to form long chain polyamides.
Normally, the salt solution is prepared containing stoichiometric amounts of diamine and diacid (which is about 56% diacid and 44% diamine by weight). Normally also, the salt solution is prepared to contain about 50% or so by weight solute content, and is then stored temporarily or is transferred to a prepolymerization vessel to await polymerization. It is concentrated to 70-80% solute by evaporating water before polymerization begins. The reason the solution is not made up at 70-80% solute concentration orginally is because at 70-80% solute concentration, the solution must be maintained at about 130.degree.-160.degree. C. to keep the solute in solution. At these temperatures discoloration and some polymerization occurs.
Thus, it has been a goal of researchers in the field to increase the solute concentration of these solutions prior to polymerization without having to maintain them at high temperatures. If the solute concentration can be increased less water will be present, thus reducing energy needs for water removal during the concentrating step. Moreover, heretofore, the solutions could not be concentrated beyond about 70-80% solute by weight because at the temperatures required to concentrate the salt solution further, polymerization began.
Thus the polymerization reaction heretofore was being conducted on salt solutions containing about 20-30% water by weight. This was not advantageous for several reasons. First the polycondensation reaction between diacids and diamines to form polyamides also forms water, and inasmuch as the reaction is an equilibrium reaction any water present must be removed before polymerization can proceed. Doing so increases the space-time of the polymerization step and leads to much greater energy requirements.